It has come to be recognized that global warming caused by an increase in carbon dioxide concentration driven by the use of a large amount of fossil fuels and an increase in energy demands driven by population growth have posed problems of annihilation of the human species. In recent years, therefore, studies are being energetically made for the utilization of sunlight that is infinite and free from the occurrence of harmful substances. For utilizing the above sunlight that is a clean energy source, there are practically used inorganic solar cells for residential buildings, such as a solar cell of single crystal silicon, polycrystal silicon, amorphous silicon, cadmium telluride and indium copper selenide.
However, these inorganic solar cells have defects. For example, silicon-based solar cells require silicon having very high purity, and naturally, the purification step thereof is complicated and requires a large number of processes, and a production cost is high. Besides these, a lightening of the cells is also required, and in particular, disadvantageously, it takes a long period of redemption for users for all the cost. They have problems that hinder the spread thereof.
On the other hand, many solar cells using organic materials have been proposed. The organic solar cells include a Schottky type photoelectric conversion device having a junction formed by a p-type organic semiconductor and a metal having a small work function and a hetero-junction type photoelectric conversion device having a junction formed by a p-type organic semiconductor and an n-type inorganic semiconductor or a junction formed by a p-type organic semiconductor and an electron-accepting organic compound. The organic semiconductor used contains a material selected from synthetic dyes or pigments such as chlorophyll, perylene, etc., electrically conductive polymer materials such as polyacetylene, etc., or composite materials of these. A thin film is formed from any one of these materials by a vacuum vapor deposition method, a casting method, a dipping method, on the like to constitute cell materials. The organic materials have advantages that their cost is low and that a large area is easily formed, while they have problems that many of them exhibit a conversion of 1% or less, and that they are poor in durability.
Under the circumstances, a solar cell that exhibits excellent characteristics has been reported by Dr. Michael Graezel of Switzerland, etc., (for example, see Non-Patent Document 1). Further, this Document also discloses materials and production technique, which are necessary for producing the cell. This solar cell is called a dye-sensitized solar cell or Graezel type solar cell, and it is a wet solar cell using, as a working electrode, a porous thin film of titanium oxide spectrally sensitized with a ruthenium (Ru) complex. This system has the following advantages; It is not required to purify a semiconductor of a less expensive oxide such as titanium oxide until it has a high purity, so that the cells are less expensive, and light that can be utilized covers up to a broad visible light region, so that sunlight containing a large quantity of visible light components can be effectively converted to electricity.
However, due to the use of Ru that is a resource-constrained noble metal, the reliable supply of Ru complex cannot be expected when dye-sensitized solar cells are practically used in the near future. Further, since Ru complex is expensive due to resource constraint, the improvement is required in view of mass production. For overcoming these problems, there have been made a variety of proposals for at least partially replacing Ru complex with less expensive organic dyes. As examples therefor, various melocyanine dyes, cyanine dyes, 9-phenylxanthene dyes, coumarin dyes, etc., have been disclosed, while they are considerably inferior to Ru complex in photoelectric conversion efficiency. Also, they have problem about stable adsorption to a semiconductor, and most of them have little practicability (for example, see Patent Documents 1-4).
Recently, organic dyes having high photoelectric conversion efficiency comparable to that of Ru complex have been recently disclosed as dye-sensitizer for dye-sensitized solar cells (for example, see Patent Documents 5, 6 and 11). These organic dyes are compounds that are classified into a melocyanine dye. Melocyanine dyes have a structure in which a unit having an electron-donating substituent (to be referred to as “electron-donor unit” hereinafter) and a unit having an electron-accepting substituent (to be referred to as “electron acceptor unit” hereinafter) are bonded through conjugated double bonds. When a melocyanine dye is used as a dye-sensitizer for a dye-sensitized solar cell, the dye has an acidic group that promotes the adsorption to a semiconductor onto its electron acceptor unit in general. However, the organic dyes described in Patent Documents 5, 6 and 11 cannot adsorb to semiconductors stably in practical use. Most of them hence have the property with a problem that the dye is re-dissolved in an electrolyte with the passage of time when a dye-sensitized solar cell is stored.
There have been disclosed trials to improve adsorption stability to a semiconductor in order to overcome the above re-dissolving problem and to improve stability (durability) with the passage of time (for example, see Patent Documents 7-10 and 12).
In Patent Document 7, two molecules of melocyanine dye are bonded together in electron donor unit to form a dye dimer, so that the adsorption stability is improved. However, the dye dimer is very insoluble to any organic solvent, therefore which it is difficult to purify the dye dimer, and the photoelectric conversion efficiency of the cell applied it thereof is insufficient.
Patent Document 8 proposes a melocyanine dye having two acidic groups which promote adsorption to semiconductor in a specific site of an electron acceptor unit. However, the synthesis of the electron acceptor unit having two acidic groups is complicated, and it has been difficult to produce the dye in a large amount.
Patent Document 9 proposes a melocyanine dye having one adsorption-promoting acid group in an electron donor unit and one adsorption-promoting acid group in an electron acceptor unit. Since, however, the one acidic group is present in the vicinity of an electron-donating substituent, the injection efficiency of electrons from a photo-excited melocyanine dye to a semiconductor is decreased, and therefore the photoelectric conversion efficiency of the cell is insufficient.
Patent Document 10 proposes a melocyanine dye in which two electron acceptor units are connected to one electron doner unit. However, the dye molecules have low symmetry, and the injection efficiency of electrons from a photo-excited melocyanine dye to a semiconductor is decreased, so that the photoelectric conversion efficiency of the cell is not so high.
In Patent Document 12, two molecules of a melocyanine dye are bonded together in an electron acceptor unit portion to form a dye dimer, so that the adsorption stability is improved. This dye dimer is excellent in both adsorption stability to a semiconductor and photoelectric conversion efficiency. Since, however, the synthesis of a dye intermediate is very complicated, it is difficult to produce the dye in a large amount.
From the viewpoint of production of more practical solar cells, therefore, there are demanded further improvements in photoelectric conversion efficiency, durability and facilitation of the production.
[Prior Art Documents]
[Patent Documents]
                [Patent Document 1] JP 11-238905 A        [Patent Document 2] JP 2001-76773 A        [Patent Document 3] JP 10-92477 A        [Patent Document 5] Japanese Translation Version No. 2002/045199 of PCT application        [Patent Document 6] Japanese Translation Version No. 2004/011555 of PCT application        [Patent Document 6] JP 2005-19252 A        [Patent Document 7] JP 2006-286609 A        [Patent Document 8] JP 2007-48672 A        [Patent Document 9] JP 2007-48680 A        [Patent Document 10] JP 2007-95584 A        [Patent Document 11] JP 2007-115673 A        [Patent Document 12] JP 2008-16383 A[Non-patent Document]        
[Non-patent Document 1] Nature, 353, 737 (1991)